Methods of dust charging are known, in which the dust is charged with corona discharge in electric field. Corona discharge is generated between a lengthy electrode (usually thin wire) and a passive electrode. Discharge electrode in located between the passive electrodes or next to them.
The airflow direction in such devices as U.S. Pat. No. 6,635,106 and U.S. Pat. No. 5,466,279 is perpendicular to the discharge electrode axis and to the direction of the electric field lines.
The airflow direction in U.S. Pat. No. 6,497,754 is perpendicular to the discharge electrodes axis; however it is parallel to the electric field lines between the discharge and passive electrodes.
In the above-mentioned patents, dust collection is effected by way of electric field generation between flat conducting parallel plates. In the above-mentioned patents, charging and collection of dust is effected inside the airflow moving in a single direction.
Major drawbacks of the mentioned devices stem from the short time of dust charging that occurs in a strong electric field with high concentration of ions.
The first drawback is the low efficiency of charging, particles with diameter smaller than 2 μk are charged not under the effect of the electric field but mainly as a result of time in a consuming ions diffusion process.
In diffusion charging the particles charge is proportional to their diameter.
Another drawback of the known devices is the high level of ozone produced as a byproduct of corona discharge during dust charging.
In these patents, non-coincidence of the discharge electrode axis with the airflow direction results in one more drawback, in that the discharge electrode surface is contaminated by dust particles in the form of hair which catch the electrode. This results in the need in pre-filters or periodic cleaning of discharge electrodes.
Since the efficiency of the dust collection is proportional to the length of collecting electrodes, in order to improve filter efficiency the length of collection electrodes and consequently the length of the entire filter must be increased.
In many cases this restricts the use of such filters, e.g. in ceiling ventilation systems where the filter length is restricted by the inter-ceiling space.